Fluoride containing chemistries have been used for many years with prime silicon wafers (wafers that have not yet undergone ion implantation or device construction) in the semiconductor industry. Normally the fluoride chemistry (usually dilute hydrofluoric acid) is used as the last process step in the sequence called “RCA rinses”. The substrate is often contaminated from previous process steps with monolayer amounts of metal, anions and/or organic contaminants or surface residues (particles). These contaminants have been shown to have a significant impact on the electrical integrity of simple test device structures and these structures need to be cleaned efficiently without impairing their integrity. Such cleaning methods could include techniques discussed in the technical literature, for example, mt. Conf. On Solid State Devices and Materials, 1991, pp. 484-486 or Kujime, T. et al., Proc. of the 1996 Semi. Pure Water and Chemicals, pp. 245-256 and Singer, P., Semi. International, p. 88, Oct. 1995.
Patents that teach methods for cleaning prime wafers with low pH solutions include U.S. Pat. Nos. 5,560,857 and 5,645,737; 5,181,985; 5,603,849; 5,705,089.
Using fluoride chemistries (usually HF) as a final RCA cleaning step will cause the silicon wafer surface to be in a hydrophobic state (the surface is covered with Si—H groups) which will repel water. During this step a certain proportion of the wafer surface is dissolved (removed). Unless the conditions are carefully monitored (time, temperature, solution composition) the substrates can be damaged, as reported by Rafols, C. et al., J. Electroanalytic Chem. 433. pp. 77-83, 1997. Numerous compositions combine water and organic solvents. The water concentration in these solutions is very critical. Silica oxide has an etch rate of 21 Å/min (@ 25° C.) in HF/water, but in isobutanol the rate was reduced to 2.14 Å/min and even lower in acetone (an aprotic solvent) the rate was only 0.12 Å/min, as reported at NSF/SRC Eng. Res. Center, Environmentally Benign Semiconductor Manufacturing, Aug. 5-7, 1998, Stanford University.
After the Front End of Line (FEOL) cleaning process the wafer proceeds to the typical Back End of Line (BEOL) manufacturing process for semiconductor devices, in which the devices might be dynamic random access memories (DRAMs), static random access memories (SRAMs), logic, electrically programmable read only memories (EPROMs), complementary metal on silicon (CMOS), and the like. Etching fabrication technology using chemical reactions (liquid or plasma) has been used as a method of forming a wiring structure on such semiconductor substrates.
A photoresist film is deposited on the wafer to form a mask, then a substrate design is imaged on the film layer, baked, and the undeveloped image is removed with a developer. The remaining image is then transferred to the underlying material through etching (either a dielectric or metal) with reactive etching gases promoted with plasma energy.
The etchant gases selectively attack the unprotected area of the substrate. Liquid or wet etching chemistries have been used extensively over the years to etch metals, oxides and dielectrics. These chemistries can be very aggressive and can result in isotropic etching (etching equally in all directions).
Increasingly, plasma etching, reactive ion etching or ion milling are used, and such etching processes produce undesirable by-products from the interaction of the plasma gases, reacted species and the photoresist. The composition of such by-products is generally made up of the etched substrates, underlying substrate, photoresist and etching gases. The formation of such by-products is influenced by the type of etching equipment, process conditions and substrates utilized. These by-products are generally referred to as “sidewall polymer,” “veil” or “fences” and cannot be removed completely by either oxygen plasma or conventional solvents. Examples of alkaline/solvent mixture types of photoresist strippers which are known for use in stripping applications include dimethylacetamide or dimethylformamide and alkanolamines as described in U.S. Pat. Nos. 4,770,713 and 4,403,029; 2-pyrrolidone, dialkylsulfone and alkanolamines as described in U.S. Pat. Nos. 4,428,871, 4,401,747, and 4,395,479; and 2-pyrrolidone and tetramethylammonium hydroxide as described in U.S. Pat. No. 4,744,834. Such stripping compositions, however, have only proven successful in cleaning “sidewall polymer” from the contact openings and metal line etching in simple microcircuit manufacturing involving a single layer of metal process when the metal structure involves mainly Al—Si or Al—Si—Cu and the “sidewall polymer” residue contains only an organometallic compound with aluminum.
If etching residue is not removed from the substrate, the residue can interfere with subsequent processes involving the substrate. The need to effectively remove etching residue and photoresist from a substrate becomes more critical as the industry progresses into submicron processing techniques. The requirement for cleaning solutions that remove all types of residue generated as a result of plasma etching of various types of metals, such as aluminum, aluminum/silicon/copper, titanium, titanium nitride, titanium/tungsten, tungsten, silicon oxide, polysilicon crystal, etc., while not corroding the underlying metal presents a need for more effective chemistry in the processing area. The effect of poor cleaning results in low device yield, low device reliability, and low device performance.
Also, if the components in these residues are not removed or neutralized in some manner then the residues will absorb moisture and form acidic species that can corrode the metal structures. The resultant acid corrodes wiring materials to bring about an adverse effect such as an increase in electrical resistance and wire disconnection. Such problems frequently occur, in particular in aluminum and aluminum alloys generally used as wiring material. The wafer substrate in contact with acidic materials, if not controlled, can destroy the metal structures. Following completion of the etching operation it is necessary that the post-etch resist mask be removed from the protective surface to permit finishing operations.
It is desirable to develop an improved cleaning composition to remove the organic polymeric substance from a coated inorganic substrate without corroding, dissolving or dulling the metal circuitry or chemically altering the wafer substrate.
Sidewall residues have been removed with either acidic organic solvents or alkaline organic solvents. The acidic solvents are generally composed of phenolic compounds or chloro-solvent and/or an aromatic hydrocarbon and/or alkylbenzenesulfonic acids. These formulations generally need to be used at temperatures up to and beyond 100° C. These chemistries normally need to be rinsed with isopropanol.
In addition, stripping compositions used for removing photoresist coatings and cleaning compositions for removing post-etch residue have for the most part been highly flammable, generally hazardous to both humans and the environment, and comprise reactive solvent mixtures exhibiting an undesirable degree of toxicity. Moreover, these compositions are not only toxic, but their disposal is costly since they might have to be disposed of as a hazardous waste. In addition, these compositions generally have severely limited bath life and, for the most part, are not recyclable or reusable.
The photoresist around the contact hole of common interlayer dielectrics, TEOS (tetraethylorthosilicate) and boron phosphosilicate glass (BPSG), which are commonly used in ultra large scale integration (ULSI) structures for better conformity of step coverage, is usually removed with HF solutions. It is not uncommon for the HF to also attack the dielectric material. Such attack is not desirable (see Lee, C. and Lee, S. C., Solid State Electronics, 41, pp. 921-923 (1997)). Accordingly, a need exists for a more environmentally friendly stripping and cleaning formulation.
Dilute hydrofluoric acid solutions can under certain conditions remove the sidewall polymers by aggressively attacking the via sidewall of the dielectric and therefore changing the dimensions of the device, as taught by Ireland, P., Thin Solid Films, 304, pp. 1-12 (1997), and possibly the dielectric constant. Previous chemistries that contain HF, nitric acid, water and hydroxylamine are aggressive enough to etch silicon, as taught by U.S. Pat. No. 3,592,773 issued to A. Muller. Recent information also indicates that the dilute HF solutions can be ineffective for cleaning the newer CFx etch residues, as taught by K. Ueno et al., “Cleaning of CHF3 Plasma-Etched SiO2/SiN/Cu Via Structures with Dilute Hydrofluoric Acid Solutions,” J. Electrochem. Soc., vol. 144, (7) 1997. Contact holes opened on to the TiSi2 have also been difficult to clean with HF solutions since there appears to be an attack of the underlying TiSi2 layer. There may also be difficulty with mass transport of the chemicals in the narrow hydrophilic contact holes, as taught by Baklanov, M. R. et al., Proc. Electrochem. Soc., 1998, 97-35, pp. 602-609.
Recently, fluoride-based chemistries have been used in limited cases to remove post etch residues. Many of these compositions contain fluoride components, specifically hydrogen fluoride. In addition, these compositions might contain strong caustic chemicals (choline-derivatives, tetraalkyl ammonium hydroxide, ammonium hydroxide) such as disclosed in U.S. Pat. No. 5,129,955; U.S. Pat. No. 5,563,119; or U.S. Pat. No. 5,571,447, or might use a two-phase solvent system, which contains one phase with hydrofluoric acid and water while a second phase contains a nonpolar organic solvent (ketones, ethers, alkanes or alkenes) (U.S. Pat. No. 5,603,849). Other formulations include hydroxylamine and ammonium fluoride (U.S. Pat. No. 5,709,756, issued to Ward). Additional examples include quaternary ammonium salt and fluoride based compositions, as disclosed in published European Application 0662705, and organocarboxylic ammonium salt or amine carboxylate and fluoride based compositions, as disclosed in U.S. Pat. No. 5,630,904.
Other methods for cleaning metal and metal oxide residues on wafers include spraying water vapor into the plasma ashing chamber followed by introducing fluorine containing gases (hydrofluoric acid) (U.S. Pat. No. 5,181,985) or a liquid containing hydrofluoric acid, ammonium fluoride and water with a pH between 1.5 to less than 7.
Some chemistries have also included chelating agents to help remove ionic and anionic contamination from the wafer surface (PCT US98/02794) but chelating agents such as citric acid, gallic acid, and catechol among others, can be aggressive toward the aluminum oxide that covers the Al metal lines. Studies by Ohman and Sjoberg show that the strong complexing ability of citric ions can increase the aluminum oxide solubility and thereby expose the metal to further corrosion, by factors of 166 and 468 at pH 5 and 6 (see Ohman et al., J. Chem. Soc., Dalton Trans. (1983), p. 2513).
Other resist-remover chemistries, such as those in U.S. Pat. No. 5,792,274, have included a salt of hydrogen fluoride combined with a water-soluble organic solvent and water at a pH of 5 to 8. However, no mention is made of the use of ammonium hydrogen fluoride (also known as ammonium bifluoride), which provides greater stability than ammonium fluoride, or the use of a synergistic mixture of co-solvents or basic amine compounds with DMSO and a fluorinated compound.
U.S. Pat. No. 6,048,406 issued Apr. 11, 2000 to Misra et al. entitled “Benign Method for Etching Silicon Dioxide” teaches using an aqueous solution of ammonium hydrogen fluoride ((NH4)HF2) as an alternative to hydrofluoric acid because it is more benign for wet etching silicon oxide. However, there is no teaching of the use of a formulation that can remove photoresist or an etching residue. Also, there is no teaching of adding a synergistic mixture of co-solvents or basic amine compounds.
U.S. Pat. No. 5,885,477 issued Mar. 23, 1999 to Rasmussen et al. entitled “Silicon Dioxide Etch Process Which Protects Metal” teaches using an aqueous solution of ammonium fluoride and hydrofluoric acid with a salt to etch silicon oxide while minimizing corrosion. However, there is no teaching of the use of ammonium hydrogen fluoride, co-solvents or basic amine compounds.
U.S. Pat. No. 4,508,591 issued Apr. 2, 1985 to Bartlett et al. entitled “Polymethyl Methacrylate Compatible Silicon Dioxide Complexing Agent” teaches using ammonium fluoride and citric acid to etch silicon dioxide. However, as with Rasmussen et al., there is no teaching of the use of ammonium hydrogen fluoride, co-solvents or basic amine compounds. Nor is there any teaching to use such a formulation for removing etch residue or photoresist.
Accordingly, there exists a need to develop improved silicon dioxide etchant and photoresist and post-etch residue remover for a variety of unwanted materials from a wide variety of substrates. Particularly in the field of integrated circuit fabrication, it should be recognized that the demands for improved removal performance with avoidance of attack on the substrates are constantly increasing. This means that compositions that were suitable for less sophisticated integrated circuit substrates may not be able to produce satisfactory results with substrates containing more advanced integrated circuits in the process of fabrication. These compositions should also be economical, environmental friendly and easy to use.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as at the priority date of any of the claims.
Throughout the description and claims of the specification the word “comprise” and variations thereof, such as “comprising” and “comprises”, is not intended to exclude other additives, components, integers or steps.